The present invention generally relates to phased array antennas and, more particularly, to detection of calibration probe movements or displacements relative to a phased array antenna.
Phased array antennas have many applications in the fields of communications and remote sensing, and are widely used, for example, on spacecraft such as communications satellites and remote sensing satellites. A phased array antenna typically includes a number of antenna elements arranged in a planar array configuration. The amplitudes and phases of the electromagnetic radiation of the antenna elements may be coordinated as a specific distribution of amplitudes and phases among the elements to achieve antenna performance characteristics for the phased array antenna as a whole. For example, the antenna radiation can be formed into a beam, the beam pattern can be adjusted, the beam pointing direction can be adjusted or even rapidly scanned, and sidelobe level and shape can be controlled.
The performance of phased array antennas, for example, beam pointing and sidelobe level, can be adversely affected by element amplitude and phase errors relative to the desired array amplitude and phase distribution. Such amplitude and phase errors can be caused by variation in the array electronic componentsxe2x80x94such as low noise amplifiers, solid state power amplifiers, mixers, phase shifters, and variable attenuatorsxe2x80x94over the lifetime of the satellite. To detect and correct for electronic component performance changes, phased array antennas typically include a calibration system with an external source (for receive arrays) or receiver (for transmit arrays) for which the array antenna has a signature response. The calibration system can greatly improve the performance and reliability of the phased array antenna.
The calibration system may use a set of probes that are embedded in the array. Alternatively, the calibration system may more simply use a single calibration probe that is separated a distance from the array. For satellite systems a single calibration probe may be preferable because it is generally lighter and less complicated than a set of embedded calibration probes. The calibration probe may be on the groundxe2x80x94for ground calibrationxe2x80x94or may be located on the satellite for on-board calibration. In either case, the geometric relationship between the calibration probe and the array is crucial to the performance of the calibration system.
On-board calibration has several advantages over ground calibration. For example, the larger signal-to-noise ratio using on-board calibration leads to faster, more accurate measurements. Also, for example, using on-board calibration there is no need to compensate for doppler frequency shifts caused by motion between the satellite and points on the earth, and there are no atmospheric effects. A problem, however, with on-board calibration is that launch loads, i.e., forces due to spacecraft accelerations during launching, and thermal effectsxe2x80x94such as material distortions, i.e., expanding/contracting, due to changes in temperature or temperature gradientsxe2x80x94can affect the structure that holds the array and calibration probe and cause changes in the geometric relationship between the calibration probe and the array. Changes in the geometry between the calibration probe and the array can cause errors in the calibration measurement resulting in antenna beam pointing errors. Beam pointing errors can be corrected, however, if the change in geometry is known.
As can be seen, there is a need for detecting changes in the geometric relationship between the calibration probe and the array for phased array antennas. Moreover, there is a need for detecting changes in the geometric relationship between the calibration probe and the array for phased array antennas for on-board calibration of phased array antennas on spacecraft such as communication and remote sensing satellites.
In one aspect of the present invention, a method for detecting calibration probe displacement for a phased array antenna includes steps of: creating a gold standard set of antenna element phases of the phased array antenna; determining a set of element phase sensitivities of the phased array antenna; measuring a set of antenna element phases relative to array displacement of the phased array antenna; and forming a set of equations using the gold standard set of antenna element phases, the set of element phase sensitivities, and the set of antenna element phases relative to array displacement. The set of equations has an array displacement vector x as unknown; and solving the set of equations for the array displacement vector x provides the location and orientation of the calibration probe displacement.
In another aspect of the present invention, a method for detecting calibration probe displacement relative to a phased array antenna, includes a step of creating a gold standard set of antenna element phases including measuring a gold standard antenna element phase of several array elements of the phased array antenna with a calibration probe at a nominal position. The method also includes a step of determining a set of element phase sensitivities of the phased array antenna, including: measuring baseline antenna element phases for several array elements with a calibration probe at a nominal position; displacing the calibration probe a known amount and direction to a first displaced position; and measuring displaced antenna element phases for several array elements with the calibration probe at the first displaced position. The method also includes a step of measuring a set of antenna element phases relative to array displacement including measuring antenna element phases relative to array displacement of the array elements of the phased array antenna with the calibration probe at a second displaced position. The method further includes steps of forming a set of equations using the gold standard set of antenna element phases, the set of element phase sensitivities, and the set of antenna element phases relative to array displacement, the set of equations having an array displacement vector x as unknown; and solving the set of equations for the array displacement vector x.
In still another aspect of the present invention, a method for in-flight detection of relative displacement between a calibration probe on-board a spacecraft and a phased array antenna on-board the spacecraft, includes a step of creating a gold standard set of antenna element phases including measuring a gold standard antenna element phase of several array elements of the phased array antenna with a calibration probe at a nominal position under controlled conditions.
The method also includes a step of determining a set of element phase sensitivities of the phased array antenna under controlled conditions, including: measuring a baseline antenna element phase for several array elements with a calibration probe at a nominal position; displacing the calibration probe a known amount and direction to a first displaced position; measuring a first displaced antenna element phase for several array elements with the calibration probe at the first displaced position; subtracting the baseline antenna element phase from the first displaced antenna element phase and dividing by the known amount; rotating the calibration probe a known angle and direction to a second displaced position; measuring a second displaced antenna element phase for several array elements with the calibration probe at the second displaced position; subtracting the baseline antenna element phase from the second displaced antenna element phase and dividing by the known angle.
The method also includes a step of measuring a set of antenna element phases relative to array displacement by using a calibration system while the spacecraft is in flight including measuring antenna element phases relative to array displacement of the array elements of the phased array antenna with the calibration probe at a third displaced position.
The method further includes steps of forming a set of equations using the gold standard set of antenna element phases, the set of element phase sensitivities, and the set of antenna element phases relative to array displacement, the set of equations having an array displacement vector x as unknown, wherein the array displacement vector x determines a location and orientation of the third displaced position; and solving the set of equations for the array displacement vector x.
In yet another aspect of the present invention, a method for in-flight detection of relative displacement between a calibration probe on-board a spacecraft and a phased array antenna on-board the spacecraft includes a step of creating a gold standard set of antenna element phases including measuring a gold standard antenna element phase Gp1 of an array element of the phased array antenna with a calibration probe at a nominal position under controlled conditions.
The method also includes a step of determining under controlled conditions a set of element phase sensitivities for the array element, including a xcex94x_sensitivity1 , a xcex94y_sensitivity1 , a xcex94z_sensitivity1 , an rx_sensitivity1 , and an ry_sensitivity1 , including: measuring a baseline antenna element phase for several array elements with a calibration probe at a nominal position; displacing the calibration probe a known amount and direction to a first displaced position; measuring a first displaced antenna element phase for several array elements with the calibration probe at the first displaced position; subtracting the baseline antenna element phase from the first displaced antenna element phase and dividing by the known amount; rotating the calibration probe a known angle and direction to a second displaced position; measuring a second displaced antenna element phase for several array elements with the calibration probe at the second displaced position; subtracting the baseline antenna element phase from the second displaced antenna element phase and dividing by the known angle.
The method also includes a step of measuring a set of antenna element phases relative to array displacement by using a calibration system while the spacecraft is in flight including measuring an antenna element phase Ep1 relative to array displacement of the array element of the phased array antenna with the calibration probe at a third displaced position.
The method also includes a step of forming a set of equations using the gold standard set of antenna element phases, the set of element phase sensitivities, and the set of antenna element phases relative to array displacement, the set of equations having an array displacement vector x=(xcex94x, xcex94y, xcex94z, rx, ry) as unknown, where the array displacement vector x determines a location and orientation of the third displaced position, and the set of equations includes the equation:
(xcex94x_sensitivity1 xc2x7xcex94x)+(xcex94y_sensitivity1 xc2x7xcex94y)+(xcex94z_sensitivity1 xc2x7xcex94z)+(rx_sensitivity1 xc2x7rx)+(ry_sensitivity1 xc2x7ry)=(Ep1xe2x88x92Gp1).
The method further includes steps of ordering the set of equations and writing the set of equations in matrix notation as: Ax=(Epxe2x88x92Gp); and solving the set of equations for the array displacement vector x.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.